The invention relates to a device for actuating a gas exchange of an internal combustion engine including a hydraulic compensating structure.
Electromagnetic actuators for actuating the gas exchange valves generally have two operating magnetsxe2x80x94one opening magnet and one closing magnet between whose pole faces an armature is arranged such that it can be displaced coaxially with the valve axis. The armature acts on a valve stem of the gas exchange valve either directly or via an armature tappet. In the case of actuators according to the principle of the mass oscillator, a prestressed spring mechanism acts on the armature. Usually, two prestressed valve springs, an upper valve spring of which biases the gas exchange valve in the opening direction and a lower valve spring of which biases the gas exchange valve in the closing direction, are used as the spring mechanism. When the magnets are not energized, the armature is held by the valve springs in a position of equilibrium between the two magnets. The valve springs can be arranged together on one side or separately from each other on opposite sides of the actuator.
When the actuator is first activated, either the closing magnet or the opening magnet is briefly overexcited or the armature is excited at its resonance frequency by an oscillation excitation routine, in order to move the armature out of the position of equilibrium. In the closed position of the gas exchange valve, the armature bears against the pole face of the energized closing magnet and is held by it. The closing magnet compresses further the valve spring, which acts in the opening direction. In order to open the gas exchange valve, the closing magnet is deenergized and the opening magnet is energized. The valve spring which acts in the opening direction then accelerates the armature beyond the position of equilibrium, with the result that the latter is attracted by the opening magnet. The armature impacts against the pole face of the opening magnet and is held firmly by the pole face. In order to close the gas exchange valve again, the opening magnet is deenergized and the closing magnet is energized. The valve spring which acts in the closing direction accelerates the armature beyond the position of equilibrium toward the closing magnet. The armature is attracted by the closing magnet, impacts on the pole face of the closing magnet and is held firmly by the latter. The two valve springs are compressed to such an extent that, when the operating magnets are deenergized, the armature moves to an approximately central position between the pole faces of the operating magnets, and that at the same time, in, or shortly before, the closing position of the gas exchange valve a residual closing force from the lower valve spring acts on the gas exchange valve.
Variables which have not been taken into consideration from the beginning or which change over time, for example manufacturing tolerances of individual components, thermal expansions of different materials, differing spring stiffnesses of the upper and lower valve springs on account of manufacturing tolerances, and also settling phenomena because of aging of the valve springs etc, may result in a position of equilibrium not coinciding with an energetic central position between the pole faces or not having a predetermined position, as this position is determined by the valve springs. Furthermore, variables of this type and wear on the valve seats may lead to the armature not bearing with a constant closing force against the pole face of the closing magnet or already bearing against it before the gas exchange valve is completely closed. Hot combustion gases, which escape past valves that are not tightly closed, destroy the valve seats. Also, different thermal expansions may cause the armature to no longer bear completely against the pole face of the closing magnet when the gas exchange valve is closed. As a result, the energy requirement of the closing magnet sharply increases. Furthermore, this process is generally associated with a reduced opening stroke of the gas exchange valve, with the result that the throttling losses increase during the charge cycle and the efficiency deteriorates.
If the gas exchange valves are actuated by a camshaft, thermal expansions, seat-ring deflection, settling phenomena because of aging of the valve springs etc. may likewise lead to the gas exchange valve not closing completely.
An earlier application DE 19 647 305 C1, illustrates an electromagnetic actuator which is mounted in a floating manner in a cylinder head. The said actuator opens and closes a gas exchange valve, while an armature is moved between two electromagnets and, in the process, acts on a valve stem of the gas exchange valve. A spring mechanism is arranged between the actuator and the valve disc of the gas exchange valve, with the upper opening spring being supported on the actuator and the lower closing spring on the cylinder head. A play-adjusting element which compensates for both a positive and a negative valve play is situated between a cover plate, which is connected to the cylinder head and the actuator on the side which faces away from the gas exchange valve.
The play-adjusting element has a piston in a cylinder. The piston separates a first pressure space, which faces away from the gas exchange valve and is controlled as a function of the internal combustion engine from a second pressure space, which faces the gas exchange valve. When there is excessive pressure in the first pressure space, a non-return valve in the piston opens in the direction of the second pressure space counter to the force of a holding spring. The holding spring is designed in such a manner that the non-return valve does not open if there is no play.
The gas exchange valve should always close securely. In order to achieve this, the play-adjusting element has the tendency to constantly slowly become shorter. This is achieved by a leakage area, which is formed by a defined play between the piston and the cylinder. When load is applied, pressure medium flows from the second pressure space into the first pressure space via the leakage area. If the armature no longer comes sufficiently close to the closing magnet or if a play arises between the armature tappet and the gas exchange valve because the play-adjusting element has become far too short, rapid adjustment in the opposite direction has to take place, which is achieved by the non-return valve opening. The pressure in the second pressure space drops below that of the first pressure space, so that the non-return valve opens towards the holding spring and pressure medium flows from the first pressure space into the second pressure space until the play has been compensated. This process may last for a number of working cycles of the valve.
The iterative process providing for rapid and slow adjustment has the effect that the gas exchange valve moves continuously within an optimum play-setting range. However, when the actuator is switched off, the armature is set by the valve springs to a position of equilibrium between the two magnets. In this case, a force of the valve springs acts on the second pressure space via the actuator. The pressure in the upper pressure space, which is controlled as a function of the internal combustion engine, drops and pressure medium is discharged from the second pressure space via the leakage area between the piston and the cylinder. The play-adjusting element collapses and the actuator is displaced upwards in the direction facing away from the gas exchange valve. As a result, the position of equilibrium of the valve springs is changed. After a renewed start of the actuator, the second pressure space of the play-adjusting element has to be filled, the actuator has to be displaced in the direction of the gas exchange valve and the position of equilibrium of the valve springs set to its correct value. This process may last for a number of working cycles of the gas exchange valve and may, in particular, lead to noises, unnecessary wear and to an additional expenditure of energy.
DE 41 09 666 A1 discloses a desmodromic control of a gas exchange valve, in which an opening cam and a closing cam act on a valve stem via a cup tappet. Two play-adjusting elements are arranged in the cup tappet, specifically an upper play-adjusting element, which faces away from the gas exchange valve and a lower play-adjusting element, which faces the gas exchange valve. Using a piston/cylinder unit, the upper play-adjusting element holds a bottom part of the cup tappet on the opening cam and, as it does so, is supported in the opening direction on the valve stem by the cylinder and in the closing direction on the bottom part by the piston. The second play-adjusting element uses a second piston/cylinder unit to hold a circumferential part of the cup tappet in contact with an arm, which is driven by the closing cam. The piston is designed as an annular piston, which is supported in the closing direction on the valve stem via a securing ring. It is guided displaceably in the circumferential part, which serves at the same time as the cylinder of the second play-adjusting element. The annular piston separates a lower pressure space, which is arranged on the sided which faces the gas exchange valve, from an upper supply space, which is arranged on the side, which faces away from the gas exchange valve. Furthermore, the second play-adjusting element has a non-return valve via which pressure medium can flow from the supply space to the pressure space via an aperture in the annular piston. The non-return valve closes the aperture with a valve ball, which is biased by a prestressed helical compression spring in the direction of the supply space. Two further helical compression springs, which are arranged in the pressure space, prestress the annular piston relative to the circumferential part.
In addition to the two play-adjusting elements, a pressure relief valve or a safety valve is arranged in the cup tappet. The safety valve is a non-return valve, which closes a second aperture in the annular piston by means of a valve ball, which is biased by a prestressed helical compression spring in the direction of the pressure space.
The helical compression spring is designed with regard to its prestressing force in such a manner that the safety valve remains closed in the case of forces which occur during normal valve actuation. However, should pumping up, i.e. a continuous expansion of the second play-adjusting element, take place during the valve operation because of resonant oscillations or an excessively high lubricating oil pressure, etc., the positive control via the closing cam would enable impermissibly high valve forces or pressures to occur which can be dissipated, in the case of the proposed design, by opening the pressure relief valve. The same applies whenever, during a prolonged standstill of the internal combustion engine, the first play-adjusting element has emptied, and during starting up, the second play-adjusting element expands ahead of the first play-adjusting element and impermissibly high valve forces would occur as a result.
The object of the invention is to provide a device for actuating gas exchange valves of an internal combustion engine having a compensating element which, by means of an iterative process providing for rapid and slow adjustment, is always within an optimum setting range, and the latter is achieved as rapidly as possible after startup of the internal combustion engine.
In a device for actuating a gas exchange valve for internal combustion engines, which device has at least one compensating element, that is arranged in the force path of an actuating element of the gas exchange valve and includes a pressure space, which is formed by a piston and a working cylinder and is connected to a pressurized fluid supply space via a non-return valve and a throttling structure via which pressure medium can be discharged from the pressure space during the working. cycles, the pressure space is sealed to the outside and the compensating element has a high-pressure valve, via which pressure medium is discharged, in a throttled manner, during the working cycles of the gas exchange valve as the high-pressure valve opens when a predetermined force acts on the compensating element and closes when another predetermined force which is greater than an average force and smaller than, or equal to, a maximum force acts on the compensating element.
With the gas exchange actuating device according to the invention, the compensating element maintains its setting when the internal combustion engine is at a standstill. This can be brought about by the compensating element being blocked mechanically, electrically or hydraulically when the internal combustion engine is shut down. One simple option is for the discharging of the pressurized fluid via the leakage area to be controllable by means of a valve. The valve can be a solenoid valve which, in the deenergized state, blocks the flow through the leakage area. The said valve can be activated as a function of suitable operating parameters of the internal combustion engine, so that discharging from the pressure space is possible only at certain time slots of the actuating cycle of the gas exchange valve. Otherwise, the effect is achieved by the compensating element being blocked hydraulically when the internal combustion engine is at a standstill, thereby maintaining its setting.
The valve can be arranged upstream or downstream of the leakage area, in the direction flow. If the leakage flow is provided by a leakage area between the piston and the cylinder, the valve is expediently arranged in a discharging or return line which, between the throttle gap and a sealing ring surrounding the piston, leads to the working cylinder.
A particular embodiment of the invention is based on the recognition that, in the case of devices for actuating gas exchange valves, the force acting on a compensating element during the working cycles fluctuates cyclically between a maximum and a minimum value, because of mass forces of inertia, pressure fluctuations in the cylinder head and, in particular in the case of devices which have at least one valve spring acting on the gas exchange valve, because of the change in the tension force over a working period. These fluctuations, which occur only during the working cycles, are used by the invention to achieve a defined leakage rate during the working cycles and to suppress the tendency for the compensating element to become slowly shorter or, in particular arrangements, to become longer when the internal combustion engine is at a standstill. During an engine shutdown, the compensating element therefore maintains its setting essentially unchanged.
Slow adjustment of a desired iterative process in one direction is preferably achieved by a high-pressure valve. During the working cycles, a certain amount of pressure medium is discharged, in a throttled manner, via the high-pressure valve as the latter open cyclically when a defined force is effective on the compensating element and closes upon occurrence of another predetermined force. The forces in each case are greater than an average force and smaller than, or equal to, a maximum force on the compensating element.
Rapid adjustment of the iterative process in the opposite direction is achieved by a non-return valve. If the internal combustion engine is shut down, cyclical fluctuations of the force on the compensating element do not occur. The defined opening force of the high-pressure valve in the region of a maximum force is not achieved or, in the case of certain devices, maintained only for a short period, for example in the case of a device having a valve spring which acts in the closing direction and in which the gas exchange valve remains in the open position. If, the force acting on the compensating element is smaller than the opening force, when the internal combustion engine is at a standstill, the high-pressure valve remains closed. No pressure medium is then discharged from the pressure space, which is tightly closed to the outside. As a result, the compensating element maintains its setting. If the acting force is greater than the opening force, only a small amount of pressure medium is discharged until the closing force is reached and the high-pressure valve is closed. The setting of the compensating element is then only slightly changed.
The solution according to the invention can be used in various types of devices for actuating a gas exchange valve, such as in the case of devices which have a valve opening cam and a valve closing cam and which do not have a valve spring. The invention concept may also be used in connection with devices having an opening cam and having a valve spring acting in the closing direction, etc. However, the solution according to the invention is used particularly advantageously in connection with electromagnetic gas exchange valve control mechanisms. Electromagnetic gas exchange valve control mechanisms have an actuator which has an opening magnet and a closing magnet with pole faces between which an armature is arranged in a manner such that it can be displaced coaxially and act on a valve stem of the gas exchange valve. Furthermore, a spring mechanism having at least one prestressed valve spring, which acts in the opening direction, and at least one prestressed valve spring which acts in the closing direction acts on the gas exchange valve. If the actuator is switched off by the internal combustion engine, the armature is set to a position of equilibrium of the valve springs between the pole faces of the magnets. In this position, the compensating element is acted upon by a force which is smaller than the opening force of the high-pressure valve and is greater than the opening force of the non-return valve, with the result that pressure medium is not discharged from the tightly closed pressure space and the setting of the compensating element is maintained when the internal combustion engine is shut down. The same effect can be obtained by the solenoid valve, which can be activated electrically.
The compensating element can be a play-adjusting element, a compensating element with which the prestress of a valve spring can be set or another compensating element which is arranged in the force transfer path of an actuating element to a gas exchange valve.
Further advantages will become apparent from the following description of an embodiment on the basis of the drawings.